The present invention relates to steel cords intended in particular for the reinforcement of articles made of plastic and/or rubber, especially tire envelopes. It relates more particularly to cords for the reinforcement of the carcass of such tire envelopes.
More precisely, the invention concerns hybrid steel cords, i.e. ones comprising wires of steels of different types, the said cords having endurance greater than that of conventional steel cords for tires.
Conventional steel cords for tires have been described in numerous documents. In a known way, they consist of pearlitic (or ferritic-pearlitic) carbon steel tires, hereinafter referred to as xe2x80x9ccarbon steelxe2x80x9d, with carbon contents normally between 2% and 1.2% (% by weight), and whose diameters normally range from 0.10 to 0.50 mm (millimeters). Such wires are required to possess a very high tensile strength, generally of at least 2000 MPa and preferably more than 2500 MPa, obtained due to the structural hardening that takes place during the cold-drawing process. The wires are then assembled together in the form of cables or strands, and the steels used must therefore also possess sufficient ductility in torsion.
As is known, these steel cords are subjected to large stresses when the tires are rolling, notably to repeated bending or variations of curvature which cause the wires to rub against one another and undergo wear and fatigue (phenomena summarized by the term xe2x80x9cfatigue-frettingxe2x80x9d). Furthermore, the presence of humidity plays an important part by inducing corrosion and accelerating the above degradation process (a phenomenon known as xe2x80x9cfatigue-corrosionxe2x80x9d) compared with utilization in a dry atmosphere. All these known fatigue phenomena, hereinafter summarized as xe2x80x9cfatigue-fretting-corrosionxe2x80x9d, result in a progressive degeneration of the mechanical properties of the cords and, under the most severe rolling conditions, can affect their lifetime.
To improve the life of tire envelopes with a metallic carcass, in which repeated bending stresses can be particularly severe, patent application EP-A-648 891 proposed steel cords with improved endurance and corrosion resistance, consisting of stainless steel wires whose composition and microstructure confer upon these stainless steel wires both the tensile strength and the torsional ductility required to replace carbon steel wires; in particular, the microstructure of the stainless steel contains at least 20% and preferably at least 50% by volume of martensite.
Compared with conventional cords made with carbon steel wires, the cords made with such stainless steel wires with at least 20% by volume of martensite have improved endurance due to better resistance of the stainless steel wires to fatigue-fretting corrosion compared with the resistance shown by carbon steel-wires. This improved resistance considerably increases the tire life.
However, compared with the said conventional carbon steel wire cords, the cords according to EP-A-648 891 have the disadvantage of being expensive because of the composition of the steel and the process for obtaining the wires; the said application, moreover, suggests briefly that to reduce the cost, hybrid steel cords should be used consisting only in part of stainless steel wires with at least 20% by volume of martensite, while the remainder can consist of carbon steel wires.
The cost of these particular stainless steel wires is higher, mainly because of the additional transformation stages needed in order to obtain, by cold-drawing, a microstructure with a high martensite content. Besides, it is known that the more a stainless steel is transformed, notably by drawing, the more it hardens and the more difficult it becomes to transform at each subsequent stage; this may cause problems with the drawing dies, notably more rapid die wear, and so increase the wire-drawing costs.
All these disadvantages, taken together, of course have an adverse effect on the cost of the tires themselves.
The objective of the present invention is to reduce the above disadvantages by proposing new steel cords whose endurance is appreciably improved compared to that of conventional cords comprising only carbon steel wires, the endurance of the cords according to the invention being very close to that of the cords according to EP-A-648 891 mentioned earlier, the said cords being formed using specific stainless steel wires, but ones obtainable at definitely less cost.
The applicants found during the course of research that, surprisingly, the use of at least one stainless steel wire in a steel cord comprising carbon steel wires improves the fatigue-fretting-corrosion resistance of the carbon steel wires in contact with the stainless steel wire. The endurance properties of the steel cord itself are consequently globally improved, as also is the life of tires reinforced with such cords.
Due to this unexpected effect of the stainless steel wire, the hybrid cords of the invention can comprise a majority of carbon steel wires which bear most of the load, and only a limited number of stainless steel wires, or even just one, whose role is to improve the fatigue-fretting-corrosion resistance of the carbon steel wires by simple contact with them.
Furthermore, since the stainless steel wires no longer have to be load-bearing in contrast to those of the cords in the aforementioned application EP-A-648 891, an entirely advantageous result is that the initial stainless steel need no longer be severely transformed so as to obtain a microstructure with a high martensite content; neither is it necessary to use specific stainless steels capable of giving such a high-martensite microstructure after cold-drawing. Thus, stainless steel wires obtained by less costly processing methods can advantageously be used.
Consequently, a first object of the invention is a hybrid steel cord comprising, in contact with one or more carbon steel wire(s), at least one stainless steel wire whose microstructure contains less than 20% by volume of martensite.
A second object of the invention is the use, in a steel cord, of at least one stainless steel wire to improve by contact the fatigue-fretting-corrosion resistance of one or more carbon steel wire(s), this use being applicable with any type of stainless steel wire and not being limited in particular to a stainless steel wire whose microstructure contains less than 20% by volume of martensite.
Another object of the invention is a method for improving, in a steel cord, the fatigue-fretting-corrosion resistance of one or more carbon steel wire(s), characterized in that during the manufacture of the said cord, at least one stainless steel wire is incorporated in it by addition or substitution, such that the said stainless steel wire is in contact with the said carbon steel wire(s).
The invention also concerns the use of cords in accordance therewith for the reinforcement of articles made from plastic and/or rubber, for example pipes, belts, tire envelopes, and reinforcement plies designed notably to reinforce the crown or carcass of such envelopes.
The invention also concerns the said articles made of plastic and/or rubber themselves when reinforced with cords according to the invention, notably tire envelopes and their carcass reinforcement plies, more particularly when intended for commercial vehicles such as vans, trucks, trailers, underground vehicles, and equipment for transport, maintenance or civil engineering.